Polyimide resin has excellent heat resistance, an excellent electrical insulating property, excellent chemical resistance, and a mechanical property, and therefore is used for electric and electronic purposes. For example, the polyimide resin is used as materials of (i) an isolation film and a protective coating agent provided on a semiconductor device, (ii) a surface protective material and a base material resin for a flexible circuit substrate, an integrated circuit, and the like, and (iii) an interlayer insulating film and a protective film for a fine circuit. Particularly, in a case where the polyimide resin is used as a coating material, the polyimide resin is used as (a) a protection material made by applying an adhesive agent on a shaped product such as a polyimide film, (b) a liquid-form polyimide resin solution, and the like.
The polyimide resin solution is roughly classified into two types: one type is a polyamide acid solution, which is a solution of a polyimide resin precursor; and the other type is a polyimide solution that contains polyimide resin soluble in an organic solvent. However, the polyamide acid solution and the polyimide solution are polymer solutions of a high molecular weight polymer and have a large molecular weight and low solubility to a solvent. This makes it difficult to prepare these solutions to have a high solute concentration, and therefore it is necessary to evaporate a large amount of the solvent in order to form a coating film, for example. This renders productivity of the coating film poor. Moreover, in a case of the polyamide acid solution, which is a solution of a polyimide resin precursor, an amide bond in its structure is easily degraded. On this account, the polyamide acid solution has poor stability. Particularly, when the solution is prepared to have a high concentration, a viscosity of the solution greatly changes, unfortunately. In view of this, the demand for a polyimide resin precursor solution having good solution storage stability is increased. Further, in the case of the solution of the polyimide resin precursor, it is necessary to imidize a coating film which is formed by applying the solution. At this time, the imidization should be carried out at a heat temperature of more than 300° C. However, in a case where the solution is used, for example, as a protection agent for a flexible substrate or the like, or as an adhesive agent to be applied on a shaped product, since wiring materials cannot endure a high temperature, it is necessary to use resin that is curable at low temperatures (not more than 200° C.) so as not to cause deterioration of wiring lines.
In relation to the techniques for these polyimide resin solutions, a copolymer having excellent solution storage stability is proposed which copolymer is obtained from a polyimide and a polyimide precursor (polyamide acid) having a specific structure. The copolymer is also referred to as a poly(amide acid-imide) copolymer (see Patent Literature 1).
Further, there has been also proposed polyamide acid which is curable at low temperatures of not more than 200° and which has a specific structure (see Patent Literature 2).
Moreover, there has been proposed a method for producing a novel polyamide acid in which method the polyamide acid is controlled to have a given low molecular weight so that the polyamide acid can be stably produced so as to have a high concentration and a low viscosity, with high reproducibility (see Patent Literature 3).
Further, there has been proposed a composition for a polyimide insulating film, which composition is made by use of an imide siloxane oligomer having a half-esterified terminal (see, for example, Patent Literature 4).